Cardiac arrhythmias are an important cause of neurologic symptoms. Bradyarrhythmias and tachyarrhythmias, by disrupting blood flow to the brain and causing cerebral hypoxia, can lead to loss of consciousness (and ultimately to cerebral necrosis or death). Thus cardiac arrhythmias must be considered as a potential cause in any patient presenting with syncope. For syncopal patients in whom the substrate for reentrant ventricular arrhythmias is found (that is, patients with ventricular dysfunction), management must be extremely aggressive because their near-term risk for sudden death is high, and conservative therapy is ineffective. Embolic stroke is a common result of atrial fibrillation. Several randomized trials now indicate that anticoagulation with low-dose warfarin is necessary, whenever possible, in patients with atrial fibrillation even in the absence of underlying valvular heart disease. Ongoing studies are evaluating the efficacy of aspirin in preventing embolic events.